Semiconductor device with mushroom electrode and manufacture method thereof

ABSTRACT

A semiconductor device has: a semiconductor substrate having a pair of current input/output regions via which current flows; an insulating film formed on the semiconductor substrate and having a gate electrode opening; and a mushroom gate electrode structure formed on the semiconductor substrate via the gate electrode opening, the mushroom gate electrode structure having a stem and a head formed on the stem, the stem having a limited size on the semiconductor substrate along a current direction and having a forward taper shape upwardly and monotonically increasing the size along the current direction, the head having a size expanded stepwise along the current direction, and the stem contacting the semiconductor substrate in the gate electrode opening and riding the insulating film near at a position of at least one of opposite ends of the stem along the current direction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/726,761,which is a divisional of application Ser. No. 12/003,559 filed Dec. 28,2007, now U.S. Pat. No. 7,709,310, which is a divisional of applicationSer. No. 11/713,599 filed Mar. 5, 2007, now U.S. Pat. No. 7,335,542,which is a divisional of application Ser. No. 10/768,092 filed Feb. 2,2004, now U.S. Pat. No. 7,223,645, which is a divisional of applicationSer. No. 10/084,924, now U.S. Pat. No. 6,717,271, filed Mar. 1, 2002,which is based on Japanese Patent Applications No. 2001-236301, filed onAug. 3, 2001, and No. 2002-019361, filed on Jan. 29, 2002, the wholecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

A) Field of the Invention

The present invention relates to a semiconductor device and itsmanufacture method, and more particularly to a semiconductor devicehaving a so-called mushroom electrode and its manufacture method.

B) Description of the Related Art

The operation speed of a field effect transistor depends upon the gatelength along the current path direction. In order to speed up a fieldeffect transistor, it is desired to shorten the gate length. If theresistance of the gate electrode increases, a high speed operation ofthe transistor is restrained. In order to lower the gate electroderesistance, it is desired to set the cross sectional area of the gateelectrode to a predetermined value or larger.

These requirements can be met by a mushroom type gate electrode whichhas a limited size of a lower part and a magnified size of the upperpart. A generally upright lower part of the mushroom electrode is calleda stem and the upper part with the magnified cross sectional area iscalled a head. A mushroom gate electrode is formed by vapor-depositing agate electrode layer on a photoresist layer having a lower opening withvertical side walls and an upper expanded opening, and lifting off theresist layer.

As the aspect ratio of a vertical opening to be formed in a resist layerbecomes large, it becomes difficult to uniformly bury the lower verticalopening with a gate electrode layer. In order to mitigate thisdifficulty, it has been proposed to form an upwardly broadening loweropening of a forward taper shape in a resist layer, and vacuum-depositan upwardly broadening gate electrode stem of a forward taper shapewithout forming any void.

In forming an upward broadening gate electrode stem of a forward tapershape, it is important to reliably control a gate length and a contactcross section between semiconductor and the gate electrode in order toimprove the performance and reliability of the device. A conventionaltapering method is, however, insufficient in that a uniform openingshape and a gate electrode cross-sectional shape at the contact areabetween semiconductor and the gate electrode cannot be formed reliably.

If a field effect transistor to be formed has a gate length longer than0.15 μm, a mushroom gate electrode can be formed without any problem byforming a lower opening with generally vertical side walls in aphotoresist layer. If a device having a gate length equal to or shorterthan 0.15 μm is formed by a conventional method, a manufacture yield ofgate electrodes lowers.

It is desired to form an upwardly broadening resist opening of a forwardtaper shape for forming the stem of the gate electrode.

In forming an upwardly broadening gate electrode of a forward tapershape by a conventional method, a gate electrode stem opening is formedin a resist layer and is forwardly tapered by utilizing glasstransition. This conventional method has, however, poor controllabilityso that a uniform gate length is difficult to be set. Because of poorcontrollability, the cross section at the contact between semiconductorand the gate electrode is difficult to be controlled and an operationspeed and reliability of devices cannot be improved.

A fine gate opening for a conventional mushroom gate having a highaspect ratio is upwardly broadened by utilizing resist glass transition.This method has, however, poor controllability and is difficult toobtain a uniform opening length, i.e., gate length. Because of poorcontrollability, it is difficult to control the cross section of thecontact area between semiconductor and the gate electrode and improvethe operation speed and reliability of devices.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductordevice having a fine gate capable of being manufactured with a highyield.

It is another object of the invention to provide a method of highlyreliably manufacturing a semiconductor device with a fine gate.

It is another object of the invention to provide a semiconductor devicehaving electrodes with various characteristics, the electrodes beingmade of the same layer.

It is another object of the invention to provide a semiconductor devicemanufacture method capable of forming electrodes with variouscharacteristics by the same process.

According to one aspect of the present invention, there is provided asemiconductor device comprising: a semiconductor substrate having a pairof current input/output regions via which current flows; a firstinsulating film formed on the semiconductor substrate and having a gateelectrode opening; and a mushroom gate electrode structure formed on thesemiconductor substrate via the gate electrode opening, the mushroomgate electrode structure having a stem and a head formed on the stem,the stem having a limited size on the semiconductor substrate along acurrent direction and having a forward taper shape upwardly andmonotonically increasing the size along the current direction, the headhaving a size expanded stepwise along the current direction, and thestem contacting the semiconductor substrate in the gate electrodeopening and riding the first insulating film near at a position of atleast one of opposite ends of the stem along the current direction.

According to another aspect of the present invention, there is provideda method of manufacturing a semiconductor device, comprising the stepsof: (a) preparing a semiconductor substrate having a pair of currentinput/output regions; (b) forming an insulating layer on thesemiconductor substrate; (c) forming a resist laminate on the insulatinglayer; (d) forming an upper opening through an upper region of theresist laminate, the upper opening having a laterally broadened middlespace; (e) forming a lower opening through a lower region of the resistlaminate, the lower opening communicating the upper opening, having alimited size along a current direction, and having generally verticalside walls; (f) etching the insulating film exposed in the loweropening; (g) performing a heat treatment of the resist laminate todeform the side walls of the lower opening so that at least one ofopposite ends of the lower opening is retracted or retarded from acorresponding end of the insulating layer and that the lower opening hasa forward taper shape upwardly and monotonically increasing a size ofthe lower opening along the current direction; and (h) filling a gateelectrode stem in the lower opening and forming a head in the upperopening, the head having an expanded size along the current direction.

According to another aspect of the present invention, there is provideda semiconductor device comprising: a semiconductor substrate having aplurality of transistor regions; and a plurality of mushroom gateelectrode structures formed on the semiconductor substrate in theplurality of transistor regions, the mushroom gate electrode structurehaving a stem and a head formed on the stem, the stem having a limitedsize on the semiconductor substrate along a current direction, and thehead having a size expanded stepwise along the current direction,wherein at least some of the mushroom gate electrode structures haveeach a taper shape upwardly and monotonically increasing a size alongthe current direction, and the taper shapes have different angles indifferent transistor regions.

According to another aspect of the present invention, there is provideda method of manufacturing a semiconductor device, comprising the stepsof (a) preparing a semiconductor substrate having a plurality of elementregions; (b) forming a resist laminate on the semiconductor substrate;(c) applying an energy beam to an upper region of said resist laminatefor defining an upper opening in each of said plurality of elementregions, and applying an energy beam to a lower region of said resistlaminate in at least part of said plurality of element regions at a dosedepending on the element region; (d) forming the upper opening throughthe upper region of the resist laminate in each of the plurality ofelement regions, the upper opening having a laterally broadened middlespace; (e) forming a lower opening through the lower region of theresist laminate in each of the element regions, the lower openingcommunicating the upper opening, having a limited size along a firstdirection, and having generally vertical side walls; (f) performing aheat treatment of the resist laminate to deform the side walls of thelower opening in at least some of the element regions in accordance withdoses so that the lower opening has a taper shape upwardly andmonotonically increasing a size of the lower opening along the firstdirection; and (g) filling a conductive stem in the lower opening andforming a head in the upper opening, the head having an expanded sizealong the first direction.

According to another aspect of the present invention, there is provideda method of manufacturing a semiconductor device, comprising the stepsof: (a) preparing a semiconductor substrate having a plurality ofelement regions; (b) forming a resist laminate on the semiconductorsubstrate; (c) forming an upper opening through an upper region of theresist laminate in each of the plurality of element regions, the upperopening having a laterally broadening middle space; (d) applying anenergy beam to a lower region of the resist lamination layer in at leastsome of the element regions at a dose corresponding to each elementregion; (e) forming a lower opening through the lower region of theresist laminate in each of the element regions, the lower openingcommunicating the upper opening, having a limited size along a firstdirection, and having generally vertical side walls; (f) performing aheat treatment of the resist laminate to deform the side walls of thelower opening in at least some of the element regions in accordance withdoses so that the lower opening has a taper shape upwardly andmonotonically increasing a size of the lower opening along the firstdirection; and (g) filling a conductive stem in the lower opening andforming a head in the upper opening, the head having an expanded sizealong the first direction.

As above, a semiconductor device having mushroom gate electrodes can bemanufactured highly reliably. Even if the gate length is short, amushroom gate electrode can be formed with a high yield.

If an insulating film is used as the lowest layer of a gate electrodestructure, the semiconductor surface and metal gate electrode can beseparated by the insulating film and direct contact therebetween can beprevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1J are cross sectional views of a semiconductor substrateillustrating a method of manufacturing a semiconductor device accordingto an embodiment of the invention.

FIGS. 2A to 2D are cross sectional views of the semiconductor deviceillustrating characteristic points of the embodiment shown in FIGS. 1Ato 1J.

FIGS. 3A to 3D are cross sectional views of a semiconductor substrateillustrating a method of manufacturing a semiconductor device accordingto another embodiment of the invention.

FIGS. 4A to 4E are cross sectional views of a semiconductor substrateillustrating a method of manufacturing a semiconductor device accordingto another embodiment of the invention.

FIGS. 5A to 5E are cross sectional views of a semiconductor substrateillustrating a method of manufacturing a semiconductor device accordingto another embodiment of the invention.

FIGS. 6A to 6E are cross sectional views of resist layers and graphsillustrating the study results made by the present inventors.

FIG. 7 is a graph showing a dose dependency upon a forward taper angleillustrating the study results made by the present inventors.

FIG. 8 is a cross sectional view of a semiconductor substrateillustrating a method of manufacturing a semiconductor device accordingto another embodiment of the invention.

FIGS. 9A to 9E are plan views and cross sectional views illustratinganother embodiment of the invention.

FIGS. 10A to 10D are plan views and cross sectional views illustratinganother embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to describing the embodiments of the invention, the study resultsmade by the inventors will be described.

In order to form a fine mushroom gate electrode, it is desired to forman opening of an upwardly broadened taper shape in a resist layer. Asresist capable of forming such an opening, polymethylmethacrylate (PMMA)is used by way of example.

As shown in FIG. 6A, on the surface of a semiconductor substrate 50, aresist layer 51 of PMMA is formed to a desired thickness by spincoating. Baking is performed at a temperature near the boiling point ofresist solvent to evaporate the solvent. The resist layer 51 afterbaking is subjected to electron beam (EB) drawing and developed to forman opening 52 having generally vertical side walls.

As shown in FIG. 6B, as the developed resist layer 51 is subjected toheat treatment, the side wall of the opening 52 changes to have anupwardly broadening forward taper shape. A forward taper opening 52 x istherefore obtained.

For example, if the boiling point of PMMA solvent is about 140° C.,baking is preformed at 145° C. after resist coating and before exposure,and heat treatment is performed at 135° C. after development. In thiscase, a forward taper angle θ of about 70 degrees can be formed. Asshown in FIG. 6A, the forward taper angle is an angle less than 90degrees between the opening side wall and the substrate surface.

During the forward taper process of the resist opening, the upper partof the opening is expanded and the size of the opening at the bottomchanges. This size change (at the bottom) is dependent upon the heattreatment temperature. FIG. 6C is a graph showing an opening lengthchange with a heat treatment temperature after development. The abscissarepresents a heat treatment temperature in the unit of ° C., and theordinate represents a ratio of a change in the opening length to theopening length before heat treatment. Although the opening lengthbecomes longer at low heat treatment temperatures, it becomes shorter athigher heat treatment temperatures. In other words, an opening having alength either longer or shorter than the original length can be formed.

The graph of FIG. 6D shows a change in the taper angle with a heattreatment temperature after development. The abscissa represents a heattreatment temperature in the unit of ° C., and the ordinate represents ataper angle in the unit of degree. At a low heat treatment temperature,the taper angle is nearly 90 degrees and the effects of the forwardtaper process are not obtained. As the heat treatment temperature rises,the taper angle becomes small and the considerable effects of theforward taper process can be obtained. For example, in order to obtain aforward taper angle of about 75 degrees, heat treatment is performed atabout 133° C.

The characteristics shown in FIGS. 6C and 6D are obtained by setting aconstant temperature of baking after resist coating and before exposure.If the baking temperature is changed, the characteristics shown in FIGS.6C and 6D are changed. Generally, as the baking is performed at a highertemperature, the effects of the forward taper process obtained at thelater heat treatment are small. It can be considered that at a higherbaking temperature, bridging of resist molecules is enhanced so thatdeformation of the resist becomes difficult at the later heat treatment.In a practical case, in order to form an opening of 0.1 μm in length, itis desired to use a taper angle of 80 degrees or smaller so that theprocess yield can be maintained high.

FIG. 6E is a graph showing a difference of the forward tapering effectbetween relatively low and high temperatures of pre-baking after resistcoating and before exposure. The abscissa represents a heat treatmenttemperature in the unit of ° C., and the ordinate represents a taperangle in the unit of degrees. As the pre-baking is performed at a hightemperature, the effects of the forward taper process obtained by heattreatment after development become small. As the pre-baking is performedat a lower temperature, the effects of the forward taper processobtained by heat treatment after development become larger.

It can be understood from these characteristics that a desired openinglength change and a desired taper angle can be obtained by selecting apre-baking temperature after resist coating and before exposure and aheat treatment temperature after development.

Generally, a resist opening for a fine gate electrode is formed by EBexposure. When EB exposure is also carried out on the region adjacent tothe gate opening at such a dose level that the resist will not bedeveloped, high forward tapering effect can be obtained at a lower heattreatment temperature. This can be ascribed to a smaller molecularweight of resist whose bonds are broken upon application of an energybeam such as an electron beam.

FIG. 7 is a graph showing a change in the taper angle obtained whenauxiliary EB radiation is performed for a region near the fine gateopening. The abscissa represents a dose in the region near a fine gateopening in the unit of μC, and the ordinate represents a taper angle inthe unit of degree. As the dose is increased, the taper angle becomessmaller at the same heat treatment temperature and the large forwardtapering effect can be obtained. Since EB exposure can be selectivelyperformed in a desired region, a desired region near the gate openingsubjected to EB exposure can be changed to have a forward taper angle.

PMMA resist can be coated repetitively to form two or more PMMA resistlayers each of which can be baked at different temperature. If a lowerlevel layer is baked at a high temperature and a higher level layer isbaked at a low temperature, the effects of the high temperature bakingare given only to the lower level layer. Therefore, the lower levellayer is difficult to have a large forward taper angle, whereas theupper level layer is likely to have a larger forward taper angle becausethe upper level layer was subjected only to the low temperature baking.If the upper level layers of the laminated resist layers are baked atlower temperatures, the taper process effects become large at the upperlevel layers.

Embodiments of the invention will be described in connection with theabove-described study results.

FIGS. 1A to 1J are cross sectional views of a semiconductor substrateillustrating the manufacture processes for a semiconductor deviceaccording to a first embodiment of the invention. FIGS. 2A to 2D arecross sectional views of the semiconductor substrate illustrating thecharacteristics of the embodiment shown in FIGS. 1A to 1J.

As shown in FIG. 1A, for example, on the surface of a GaAs substrate 1,a GaAs buffer layer 2 is grown to a thickness sufficient for relaxingthe influence of dislocation of the substrate, by a growth method suchas metal organic chemical vapor deposition (MOCVD) and molecular beamepitaxy (MBE). On this GaAs buffer layer 2, an electron transit layer 3of InGaAs is grown to a thickness of, for example, 14 nm. On theelectron transit layer 3, an electron supply layer 4 of AlGaAs is grownto a thickness of about 25 nm. On the electron supply layer 4, a lowresistance layer 5 of GaAs doped with Si is grown to a thickness ofabout 50 nm. A semiconductor substrate S is therefore formed on which asemiconductor device is formed.

In order to define element regions in a conductive semiconductorsubstrate surface layer, semi-insulating regions are formed byimplanting elements such as oxygen into regions other than activeregions and inactivating donors in the ion implanted regions.

As shown in FIG. 1B, a resist layer PR1 is coated on the surface of thesemiconductor substrate S, exposed and developed to form openings forohmic electrodes. After a resist pattern PR1 with the ohmic electrodeopenings is formed, an ohmic electrode layer 11 is formed in a highvacuum vapor deposition system, the ohmic electrode layer 11 having alamination structure of an AuGe layer of about 20 nm in thickness and anAu layer of about 300 nm in thickness. As the resist pattern PR1 isremoved, the ohmic electrode layer deposited on the resist pattern PR1is also lifted off and removed. Ohmic electrodes 11 are therefore lefton the semiconductor substrate S.

As shown in FIG. 1C, an SiN film 13 of about 20 nm in thickness isformed on the semiconductor substrate S, covering the ohmic electrodes11. This SiN film 13 improves tight contactness between the substrateand a resist layer to be formed over the substrate.

As shown in FIG. 1D, a resist layer PR2 is coated on the SiN film 13,and an opening for a gate recess region is formed through the resistlayer PR2 by EB drawing. The gate recess region is, for example, a gatearea added with an area of about 0.1 μm along the gate length directionon both sides of the gate area.

After the resist pattern PR2 with the gate recess region opening isformed, by using this resist pattern as a mask, the SiN film 13 isetched by dry etching using SF₆ gas and then the low resistance GaAslayer 5 is etched by dry etching using SiCl₄ gas. The electron supplylayer 6 is therefore exposed in the gate recess region. The resistpattern PR2 is thereafter removed.

As shown in FIG. 1E, after the gate recess region is etched, an SiN film15 having a thickness of about 20 nm is deposited on the substratesurface to protect the uppermost surface and improve tight contactnesswith a resist layer to be next formed. On the SiN film 15, a PMMApositive type electron beam resist layer PR10 is spin-coated on the SiNfilm 15 to a thickness of about 300 nm and is subjected to a heattreatment for 10 minutes at 160° C. On this electron beam resist layerPR10, an alkali-soluble resist layer R is spin-coated to a thickness ofabout 600 nm, and baked for 4 minutes at 160° C. On this alkali-solubleresist layer R, a polystyrene electron beam resist layer PR20 isspin-coated to a thickness of about 200 nm, and baked for about 4minutes at 160° C.

EB drawing is performed for the electron beam resist layer PR20 todefine an opening A1 having a width of about 0.8 μm.

As shown in FIG. 1F, the exposed region A1 of the electron beam resistlayer PR20 is developed by mixed solution of methylisobutylketone (MIBK)and methylethylketone (MEK). By using the developed electron beam resistlayer PR20 as a mask, the underlying resist layer R is etched byalkaline developing liquid. With this etching, an opening is formedthrough the resist layer R, the opening being retracted by about 0.2 μmor laterally deeper from the opening of the electron beam resist layerPR20.

A gate electrode opening having a width of about 0.1 μm is defined by EBdrawing through the electron beam resist layer PR10 exposed in theopening. An EB exposed region A3 is developed by mixed solution of MIBKand isopropyl alcohol (IPA) to form a gate electrode opening through theelectron beam resist layer PR10.

As shown in FIG. 1G, by using the electron beam resist layer PR10 as amask, the exposed SiN film 15 is etched by dry etching using SF₆ gas. Asubstrate surface having a width of about 0.1 μm is therefore exposed inthe gate electrode opening having generally vertical side walls SW1.

As shown in FIG. 1H, a heat treatment is performed for about 10 minutesat a temperature which forms a forward taper and elongates the openinglength, for example, at 130° C. With this heat treatment, while theopening length becomes longer, the forward taper upward broadening theopening is formed. A gate electrode opening A4 having slanted side wallsSW2 is therefore formed in the gate electrode opening. The side wallsSW2 of the electron beam resist layer have the shape retracting from theedges of the underlying SiN film 15, for example, by 0.01 μm.

As shown in FIG. 1I, an insulating metal oxide film, e.g., TiO_(x), isdeposited in a high vacuum vapor deposition system from the upper sideof the semiconductor substrate S subjected to the forward taper process.Then, a Ti layer, a Pt layer and an Au layer are laminated to form agate electrode layer 17.

The function of the forward taper process for a gate electrode openingbefore the gate electrode layer depositing step will be described.

As shown in FIG. 2D, if the resist layer PR10 has vertical side walls,although a vapor deposition layer 17 a vapor-deposited from the upperside of the substrate S can be reliably deposited in the central area ofthe opening bottom, it becomes difficult to be deposited in theperipheral area of the opening bottom because of the shading influencesof the side walls of the resist layer PR10. The vapor deposition layerbecomes thin in the peripheral area of the opening bottom and in somecases, the surface of the substrate S is exposed in this area. If theTiO_(x) is formed in this state and a Ti layer is vapor-depositedthereon, the Ti layer may directly contact the substrate S surface inthe peripheral area of the opening bottom. As Ti contacts thesemiconductor surface, Ti and semiconductor chemically react under thecondition that an energy such as an electric field is applied, and thedesired device characteristics cannot be obtained. This phenomenon islikely to occur on the drain side to which a strong electric field isapplied.

As shown in FIG. 2B, although a insulating layer D (SiN layer 15) on thesemiconductor substrate S has generally vertical side walls, the heightof the side wall is very low. The electron beam resist layer PR 10 onthe insulating layer D has a forward taper upward extending the width ofthe opening. A vapor deposition layer deposited from the upper side isnot obstructed by the side walls of the resist layer PR10, and can begenerally uniformly deposited in the opening of the insulating layer D.The TiO_(x) layer 17 a first deposited reliably covers the exposedsubstrate S surface to prevent a metal layer to be later deposited fromcontacting the semiconductor layer.

As shown in FIG. 2C, in the gate electrode opening defined by theinsulating layer D and electron beam resist layer PR10, a TiO_(x) 17 ahaving a thickness of, for example, several nm, a Ti layer 17 b having athickness of, for example, 10 nm, and a Pt layer 17 c having a thicknessof, for example, 10 nm are sequentially formed. Thereafter, an Au layer17 d having a sufficient thickness of, for example, about 500 nm isdeposited.

As shown in FIG. 1I, in the opening defined by the laminated resiststructure, a mushroom gate electrode 17 is therefore formed.

As shown in FIG. 1J, the semiconductor substrate is immersed into heatedorganic solution to dissolve the laminated resist layers to lift off thegate electrode layer 17 deposited on the resist lamination structure. Amushroom gate electrode 17 is therefore left on the semiconductorsubstrate S.

As shown in FIG. 2A, the mushroom gate electrode obtained has thestructure that the opposite ends of the gate electrode ride theinsulating film D. As shown in FIG. 2C, the surface of the semiconductorsubstrate S exposed in the opening of the insulating film D is securelycovered with the TiO_(x) film. The semiconductor layer S and a metallayer such as a reactive Ti layer formed on the TiO_(x) layer 17 a canbe prevented from direct contact.

The gate electrode riding the insulating film D extends outward from thegate electrode in contact with the semiconductor layer. An electricfield near the opposite ends of the gate electrode can be relaxed.

With the gate electrode structure of the embodiment, the effective gateelectrode length is determined by the opening length defined theinsulating film D. The gate electrode G has a stem broader than theopening length of the insulating film D and rides the insulating film D.For example, even if an electrode having a gate length of 0.1 μm ridesthe insulating film D by 0.01 μm, generally the same electriccharacteristics of the gate electrode structure can be retained. Sincethe contact area of the gate electrode increases and the gate electrodecovers the steps, the mechanical stability of the gate electrode can beimproved.

FIGS. 3A to 3D are cross sectional views of a semiconductor substrateillustrating the semiconductor device manufacture method according toanother embodiment of the invention.

In the processes shown in FIGS. 1A to 1G, the process of depositing theSiN layer 15 after the gate recess region is formed is omitted, and theother processes are performed in similar manners.

As shown in FIG. 3A, an opening is defined by the laminated resistlayers in the gate recess region where the semiconductor substrate S isexposed. A TiO_(x) layer 17 a is vapor-deposited from the upper side ofthis structure. On the bottom of the gate electrode opening havinggenerally vertical side walls, the TiO_(x) layer 17 a is thereforedeposited.

As shown in FIG. 38, a heat treatment is performed, for example, for 10minutes at 145 c′C under the conditions that the gate electrode openinghas a forward taper and the opening length is shortened. The gateelectrode opening has a forward taper shape upward broadening itsopening, the opening length at the bottom of the opening is shortened,and the resist layer PR10 rides the deposited TiO_(x) layer 17 a.

As shown in FIG. 3C, a Ti layer, a Pt layer and an Au layer aresequentially deposited in a high vacuum state to form a gate metal layerGM.

As shown in FIG. 3D, the semiconductor substrate is immersed into heatedorganic solution to leave a gate electrode G through lift-off.

With the gate electrode structure of the embodiment, the metal gateelectrode structure GM is formed inside the surface area of theinsulating metal oxide film 17 a on the semiconductor substrate. Sincethe opposite ends of the metal gate electrode structure GM arepositioned inside the surface area of the insulating metal oxide film 17a, it is possible to prevent a direct contact between reactive metal andthe semiconductor surface.

Next, another embodiment will be described in which an additional doseis used to enhance the forward taper process.

FIG. 7 is a graph showing a taper angle dependency upon a dose obtainedby experiments made by the present inventors. If an electron beam havingan energy equal to or larger than a threshold value is applied to an EBexposure resist, the molecular weight of the resist lowers so that theresist can be developed by resist developer liquid. If the radiationamount of an electron beam is lowered to a proper value, the state thatresist has a smaller molecular weight can be realized although theresist is not developed. If such a resist layer is subjected to a heattreatment, the resist layer can have an upward broadening taper shape ata lower temperature than an electron beam is not applied.

In FIG. 7, the abscissa represents a dose of an electron beam applied toa region near the fine gate, and the ordinate represents a taper angle.The heat treatment temperature is set to such a value that a taper shapeis hardly obtained if an electron beam was not applied. As the doseincreases, the taper angle relative to the substrate surface becomessmall starting from 90 degrees. Namely, the opening side walls broadenand the taper degree becomes large.

As shown in FIG. 4A, a lamination structure of a insulating layer D, anelectron beam resist layer PR 10, a resist layer R, and an electron beamresist layer PR20 is formed on a semiconductor substrate S, in themanner similar to those processes shown in FIGS. 1A to 1J.

A gate electrode opening A3 is drawn by an electron beam E1 at apredetermined dose. For example, a gate electrode opening having a widthof 0.1 μm is EB-drawn. An auxiliary EB radiation whose energy is setequal to or lower than a development limit, e.g., about a half of thethreshold value, is applied to the region near the gate electrodeopening, in the example shown in FIG. 4A, a right region having a widthof about 0.05 μm. As shown in FIG. 4B, the electron beam resist layerPR10 is developed by mixed solution of MIBK and IPA. With thisdevelopment, although the exposed region A3 for a gate electrode isremoved, the auxiliary exposed region Ax is left. The insulating film D,e.g., an SiN film exposed on the bottom of the gate electrode opening isremoved by dry etching using, for example, SF₆ gas.

In this embodiment, two types of electron beam radiation aresequentially performed and then the development is performed.Development may be performed after the electron beam radiation isperformed, and then the auxiliary EB exposure is performed for thedeveloped resist pattern. Also, EB exposures for the upper and lowerapertures and for affording tapering can be performed through the upperresist layer at the same stage.

As shown in FIG. 4C, a forward taper heat treatment is performed for 10minutes in a temperature range allowing the opening length to beelongated, e.g., at 130° C. The region, on the left side of the opening,of the electron beam resist layer PR10 auxiliary applied with anelectron beam changes its shape to the taper shape with a priority overthe other regions. The left side wall of the electron beam resist layerPR10 is maintained being relatively less influenced.

As shown in FIG. 4D, in a high vacuum vapor deposition system, aninsulating metal oxide film (TiO_(x) film), a Ti layer, a Pt layer andan Au layer are sequentially deposited in the opening to form a gateelectrode 17.

As shown in FIG. 4E, the semiconductor substrate is immersed into heatedorganic solution to leave a gate electrode G through lift-off. With thegate electrode structure of the embodiment, the gate electrode G ridesthe insulating film D only on one side thereof, e.g., on the drain side.Since the gate electrode rides the insulating film D on the drain side,an electric field in a strong electric field intensity region wherereaction is likely to progress can be relaxed. Since the insulatingmetal oxide film securely covers the semiconductor substrate S surface,a direct contact between reactive metal and the semiconductor substratecan be avoided.

Another embodiment will be described in which the forward taper shape iscontrolled by changing the baking temperature of a laminated resistlayer structure.

As shown in FIG. 5A, a semiconductor substrate S is prepared byperforming the processes similar to the first embodiment shown in FIGS.1A to 1J before the gate recess region is formed. On the surface of thesemiconductor substrate S, a insulating film D such as an SiN film isformed. Thereafter, a first electron beam resist layer PR11 is coated toa thickness of about 200 nm by using PMMA or the like, and baked, forexample, for 5 minutes at 185° C. On the first electron beam resist, asecond electron beam resist layer PR12 is coated to a thickness of about200 nm by using PMMA or the like, and baked, for example, for 5 minutesat 145° C. The lower resist lamination is constituted of a lower layerbaked at a high temperature and an upper layer baked at a lowtemperature.

On the second electron beam resist layer PR12, for example, analkali-soluble resist layer R is coated to a thickness of about 600 nm,and baked for 4 minutes at 145° C. On the alkali-soluble resist layer R,a polystyrene electron beam resist layer PR20 as an upper electron beamresist layer is coated to a thickness of about 200 nm, and baked forabout 4 minutes at 145° C.

As shown in FIG. 5B, EB drawing is performed for the upper electron beamresist layer PR20 to define an opening A1 having a width of about 0.8μm, and the upper electron beam resist layer PR20 is developed by mixedsolution of MIBK and MEK. By using the upper electron beam resist layerPR20 as a mask, the underlying resist layer R is etched by alkalinedeveloping liquid. With this etching, an opening is formed through theresist layer R, the opening being retracted by about 0.2 μm or laterallydeeper from the opening of the upper electron beam resist layer PR20.

An opening having a width of about 0.1 μm is defined by EB drawingthrough the laminated electron beam resist layers PR12 and PR11. Theresist layers are then developed by mixed solution of MIBK and IPA.After the opening is formed through the electron beam resist layers, theexposed insulating film D is dry-etched by SF₆ or the like.

As shown in FIG. 5C, a heat treatment is performed for the lowerlaminated resist layers PR12 and PR11 for about 10 minutes at atemperature which forms a forward taper, for example, at 140° C. Thefirst electron beam resist layer PR11 baked at a relatively hightemperature has a low forward taper degree, whereas the second electronbeam resist layer PR12 baked at a relatively low temperature has a largeforward taper degree. In this manner, the forward taper broadening moreat an upper position can be obtained.

As shown in FIG. 5D, similar to the above-described embodiment, in ahigh vacuum vapor deposition system, an insulating metal oxide film(TiO_(x) film), a Ti layer, a Pt layer and an Au layer are sequentiallydeposited in the opening to form a gate electrode structure 17.

As shown in FIG. 5E, the semiconductor substrate is immersed into heatedorganic solution to leave a gate electrode G through lift-off.

With the gate electrode structure of the embodiment, the lower stem ofthe fine gate has relatively vertical side walls, and the upper stem hasa forward taper upward broadening the opening. The insulating film D onthe semiconductor surface may be omitted.

Various modifications of the embodiments are possible. For example,although the insulating oxide film is used as the lowest layer of thegate electrode structure, a gate electrode structure that a Schottkymetal layer directly contacts the semiconductor surface may be formed.The cross section of a taper shape is not necessarily a straight line,but any other lines may be possible so long as they change monotonously.Although an SiN film is used as the insulating film, other insulatingfilms may be used. Instead of an insulating metal oxide film, otherinsulating films may also be used. The composition of a gate electrodeis not limited to those described earlier.

In the embodiments, an opening in a PMMA resist film is changed to havea forward taper shape. Instead, in manufacturing semiconductor devices,other resist layers may also be used whose opening shape can be adjustedwith good controllability in a temperature range where an abrupt openingshape change to be caused by glass transition or the like does not occur(in a temperature range lower than a glass transition temperature).

In forming the recess region, other methods may be used. For example, asemiconductor layer may be wet-etched, an SiN film may not used, or asemiconductor layer may be etched by using an opening for a mushroomgate.

In the embodiments, the head of a mushroom gate electrode is formed byusing three electron beam resist layers. Instead, the head of a mushroomgate electrode may be formed by using a backward taper resist layeropening in a photoresist layer or the like. In FIG. 8, a resist layerPR20 is formed on an electron beam resist layer PR10, and the resistlayer PR2 is formed with a backward taper opening downward broadeningits opening. The other structures are similar to those shown in FIG. 1F.The head of a mushroom gate electrode may be formed through milling byusing an inverted pattern.

Various semiconductor elements are formed in a semiconductor integratedcircuit. A high speed operation is required for some transistors and notrequired for other transistors. It is preferable that the gate length ofa transistor operating at high speed is short, and the gate length of atransistor operating at not so high speed is not so much required to beshort.

FIGS. 9A to 9E are plan views and cross sectional views of asemiconductor device according to another embodiment of the invention.

FIG. 9A is a schematic plan view showing the structure of asemiconductor integrated circuit device. On the surface of asemiconductor chip SP, a high speed circuit HP and a low speed circuitLP are formed.

FIG. 9B is a schematic plan view showing gate resist openings for atransistor formed in the low speed circuit LP. An opening AW is a gatestem opening formed through the uppermost to lowermost surfaces of thelaminated resist layers. An upper opening GW is an opening formed onlythrough upper layers of the laminated resist layers.

FIG. 9C is a schematic plan view showing gate resist openings for atransistor formed in the high speed circuit HP. An opening AN is a gatestem opening formed through the uppermost to lowermost surfaces of thelaminated resist layers. An opening GN is an opening formed only throughupper layers of the laminated resist layers. A region AD is a regionwhere auxiliary EB exposure is performed. Although the resist layer isnot developed by auxiliary EB exposure, the later heat treatment forms aforward taper shape upward broadening the opening.

As shown in FIG. 9D, the auxiliary EB exposure is not performed for thelow speed circuit, but it is performed only for the high speed circuit.For example, the auxiliary EB exposure is performed at an accelerationenergy of 50 keV and a dose of 20 μC.

Thereafter, a heat treatment is performed, for example, for 5 minutes at130° C. Since the average molecular weight of resist in a regionsubjected to the auxiliary EB exposure is low, this heat treatment formsa forward taper shape upward broadening its opening. The region notsubjected to the auxiliary EB exposure has no significant forward tapershape. After the gate electrode is deposited, the resist layer isremoved to lift off the gate electrode layer on the resist layer.

FIG. 9E shows the outline shapes of gate electrodes. The gate electrodeGW in the low speed circuit has a relatively long gate length and itsstem is defined by generally upright side walls. This gate electrode hasa high mechanical strength. The gate electrode GN in the high speedcircuit has a stem of a taper shape upward extending the size along thegate length direction, the gate length being defined at the bottom andbeing short. Such a gate electrode is suitable for a high speedoperation.

A semiconductor integrated circuit is formed not only with transistorsbut also with other electronic components such as capacitors and wiringlines. A mushroom structure is also applied to circuit components otherthan transistors.

FIGS. 10A to 10D show the structure of a semiconductor device accordingto another embodiment of the invention.

FIG. 10A is a schematic plan view showing the structure of asemiconductor chip SP. Similar to the structure shown in FIG. 9A, a highspeed circuit area HP and a low speed circuit area LP are disposed inthe semiconductor chip HP. In the high speed circuit area HP, atransistor Q operating at high speed and a circuit component P otherthan a transistor having a thick finger are disposed.

FIG. 10B is a schematic plan view showing resist openings for thecircuit component P in the high speed circuit area.

FIG. 10C is a schematic plan view showing gate resist openings for thetransistor Q in the high speed circuit area The shapes of these resistopenings are similar to those shown in FIGS. 9B and 9C. The resistopenings for the circuit component P have an opening PW formed throughthe upper resist layer and an opening formed through the upper and lowerresist layers. The resist openings for the transistor Q in the highspeed circuit area have an opening GN formed through the upper resistlayer and an opening AN formed in correspondence with the gate electrodestem. Auxiliary EB exposure regions AD are defined on both sides of theopening AN.

Similar to the embodiment shown in FIGS. 9A to 9E, the EB exposure isperformed for the regions AD, and thereafter the processes for heattreatment, electrode layer deposition and resist layer removal areexecuted.

FIG. 10D shows the outline shapes of the gate electrode and thickfinger. The circuit component P has a thick stem defined by generallyupright side walls and constitutes, for example, a wiring line. The gateelectrode GN of the transistor Q operating at high speed has a stem of ataper shape upward elongating the gate length.

In the embodiments shown in FIGS. 9A to 9E and FIGS. 10A to 10D, thelaminated resist layers may be either three layers shown in FIG. 1E ortwo layers shown in FIG. 8. A resist pattern may be formed by exposureand development or by etching after the laminated resist layers areformed, or it may be formed by forming the lowest resist layer,pattering it, and thereafter forming the upper resist layer.

In the above embodiments, after a broad region is exposed as shown inFIG. 1E and developed as shown in FIG. 1F, a narrow region is exposedand developed as shown in FIG. 1G. Instead, a plurality of exposures maybe performed first and thereafter the upper and lower resist layers aredeveloped.

In the above embodiments, a resist layer to be tapered is made of PMMA.PMMA has a glass transition temperature of, for example, 165° C. Assolvent of this resist material, ethyl cellosolve acetate (ECA, boilingpoint: about 170 to 180° C.), 140° C.+α), propylene glycol monomethylether acetate (PGMEA), boiling point: about 140° C.+α) and the like areknown. Even if solvent having a high boiling point is used, it ispreferable that baking and heat treatment of resist are performed at aglass transition temperature or lower of resist.

By using PGMEA as solvent, baking before exposure and heat treatmentafter development were performed in a temperature range of 120° C. to150° C. In the whole temperature range, the taper shapes were formed.From these results, it can be considered that a desired taper shape canbe obtained by performing baking and heat treatment at a glasstransition temperature or lower.

The embodiments of the invention have been described in connection withthe preferred embodiments. The invention is not limited only to theabove embodiments. It is apparent that various modifications,improvements, combinations, and the like can be made by those skilled inthe art.

1. A method of manufacturing a semiconductor device, comprising thesteps of: (a) preparing a semiconductor substrate having currentinput/output regions; (b) forming a first resist layer on thesemiconductor substrate and baking the first resist layer at a firsttemperature; (c) forming a second resist layer on the first resist layerand baking the second resist layer at a second temperature lower thanthe first temperature; (d) forming an upper resist structure on thesecond resist layer, the upper resist structure having an upper openinghaving a laterally broadening middle space; (e) forming a lower openingthrough the first and second resist layers, the lower openingcommunicating the upper opening, having a limited size along a currentdirection, and having generally vertical side walls; (f) performing aheat treatment on the semiconductor substrate at a third temperature togive a relatively small forward taper to the first resist layer and arelatively large forward taper to the second resist layer; and (g)filling a gate electrode stem in the lower opening and forming a head inthe upper opening to form a mushroom gate electrode, the head having anexpanded size along the current direction.
 2. The method ofmanufacturing a semiconductor device according to claim 1, wherein thefirst and second temperatures are set lower than a glass transitiontemperature of the first resist layer.